IEC TR 63262:2019
(Main)Performance of unified power flow controller (UPFC) in electric power systems
Performance of unified power flow controller (UPFC) in electric power systems
IEC TR 63262:2019(E) provides guidelines for applying unified power flow controllers (UPFC) in power systems. It includes letter symbols, terms and definitions, principles and configurations, design rules, performance requirements for key equipment control and protection, insulation co-ordination, system performance and tests. This technical report applies to the UPFC based on modular multi-level converter (MMC) technology, as well as UPFC based on three-level converter technology.
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IEC TR 63262 ®
Edition 1.0 2019-09
TECHNICAL
REPORT
colour
inside
Performance of unified power flow controller (UPFC) in electric power systems
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IEC TR 63262 ®
Edition 1.0 2019-09
TECHNICAL
REPORT
colour
inside
Performance of unified power flow controller (UPFC) in electric power systems
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.200; 29.240.99 ISBN 978-2-8322-7393-7
– 2 – IEC TR 63262:2019 © IEC 2019
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and symbols. 9
Terms and definitions . 9
Symbols . 10
4 Principles and configurations . 11
Basic principles . 11
UPFC configurations . 12
4.2.1 Basic structure. 12
4.2.2 UPFC configuration in single transmission line . 13
4.2.3 UPFC configuration in double transmission lines . 13
4.2.4 UPFC configuration in multiple transmission lines . 15
5 Design rules . 15
Proposal selection . 15
Parameter selection and coordination . 15
6 Performance requirements for key equipment . 16
General . 16
Voltage sourced converters (VSCs) . 16
6.2.1 General . 16
6.2.2 Three-level converters . 16
6.2.3 Modular multi-level converters (MMCs) . 17
Series transformer . 18
6.3.1 General . 18
6.3.2 Winding connection mode . 18
6.3.3 Insulation level . 19
6.3.4 Short circuit capability . 20
6.3.5 Over-excitation tolerance . 20
6.3.6 DC biasing . 20
Shunt transformer . 20
6.4.1 General . 20
6.4.2 Winding connection . 20
6.4.3 On-load voltage regulation . 21
6.4.4 DC biasing . 21
6.4.5 Harmonics and over-excitation tolerance . 21
Fast bypass switch (FBS) . 22
7 Control and protection . 22
Control system of UPFC. 22
7.1.1 Basic requirement . 22
7.1.2 Configuration requirements . 23
7.1.3 Functions of control system . 23
Protection system of UPFC . 24
7.2.1 Basic requirements . 24
7.2.2 Configuration requirements . 24
7.2.3 Functions of protection system . 24
Requirements on UPFC monitoring system . 25
Requirements on communication interfaces . 26
8 Insulation co-ordination . 26
Principles of insulation co-ordination . 26
8.1.1 General . 26
8.1.2 Insulation co-ordination procedure . 26
8.1.3 Arrester protective scheme . 27
Voltages and overvoltages in service . 27
8.2.1 Maximum operating voltage . 27
8.2.2 Sources of overvoltages . 28
Determination of the required withstand voltages (Urw) . 28
9 System performance . 30
General . 30
Steady-state performance . 30
9.2.1 General . 30
9.2.2 Steady state control requirement of transmission line power . 30
9.2.3 Steady state control requirement of reactive power compensation and
voltage control . 30
9.2.4 Overload capacity requirement . 30
Dynamic performance . 30
Fault ride-through performance . 31
10 Tests . 31
General . 31
Off-site tests of main components . 31
10.2.1 Converter valve . 31
10.2.2 Fast bypass switch (FBS) . 32
10.2.3 Transformers . 32
Onsite commissioning test . 33
10.3.1 General . 33
10.3.2 Converter energizing test . 33
10.3.3 Energizing test of series transformer . 34
10.3.4 UPFC initial operational tests . 34
10.3.5 Steady-state performance test . 34
10.3.6 Dynamic performance test . 34
10.3.7 Protection trip test . 34
10.3.8 Additional control function test . 34
10.3.9 Overload test . 34
10.3.10 Fault ride-through test of AC system . 34
Annex A (informative) Examples of typical UPFC projects . 35
A.1 Inez UPFC project structure of U.S.A. . 35
A.2 Kangjin UPFC project structure of South Korea . 35
A.3 Marcy UPFC project structure of U.S.A. . 36
A.4 Nanjing UPFC project structure of China . 36
A.5 Shanghai UPFC project structure of China . 37
A.6 Suzhou UPFC project structure of China . 37
A.7 Other information for typical UPFC projects . 38
A.8 Technical and economic evaluation for UPFC projects . 38
Annex B (informative) The difference between UPFC and other FACTS . 39
– 4 – IEC TR 63262:2019 © IEC 2019
Bibliography . 40
Figure 1 – UPFC used in a two-terminal transmission system . 11
Figure 2 –UPFC power flow schematic diagram . 12
Figure 3 – UPFC control functions . 12
Figure 4 – UPFC structure diagram . 13
Figure 5 – UPFC configuration in single transmission line VSC . 13
Figure 6 – UPFC configuration with non-common DC bus . 14
Figure 7 – UPFC configuration with common DC bus . 14
Figure 8 – Typical three-level converter topology . 16
Figure 9 – Typical MMC topology . 17
Figure 10 – Single-phase voltage waveform on the AC side . 18
Figure 11 – Typical structure of series transformer winding . 19
Figure 12 – Typical winding structure of the shunt transformer . 21
Figure 13 – Typical structure of TBS . 22
Figure 14 – UPFC protection function areas . 25
Figure 15 – Example of arresters protecting areas for a MMC-UPFC . 29
Figure A.1 – Main electrical circuit of Inez UPFC project . 35
Figure A.2– Main electrical circuit of Kangjin UPFC project [1] . 35
Figure A.3 – Main electrical circuit of Marcy UPFC project [1] . 36
Figure A.4– Main electrical circuit of Nanjing UPFC project [1] . 36
Figure A.5 – Main electrical circuit of Shanghai UPFC project [1] . 37
Figure A.6 – Main electrical circuit of Suzhou UPFC project [1] . 37
Table 1 – Arrester protective scheme for an MMC-UPFC . 27
Table 2 – Indicative values of ratios of required impulse withstand voltage to impulse
protective level . 29
Table 3 – Main test items of converter valve . 31
Table 4 – Main test items of TBS . 32
Table 5 – Main test items of transformers . 33
Table A.1 – Main parameters of typical UPFC projects [1] . 38
Table A.2 – Main parameters of transformers in Kangjin UPFC project . 38
Table A.3 – Main parameters of transformers in Nanjing UPFC project . 38
Table B.1 – Comparison of control parameters and application of each FACTS . 39
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PERFORMANCE OF UNIFIED POWER FLOW CONTROLLER (UPFC)
IN ELECTRIC POWER SYSTEMS
FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
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example "state of the art".
IEC TR 63262, which is a Technical Report, has been prepared by subcommittee 22F: Power
electronics for electrical transmission and distribution systems, of IEC technical committee 22:
Power electronic systems and equipment.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
22F/521/DTR 22F/531/RVDTR
Full information on the voting for the approval of this Technical Report can be found in the
report on voting indicated in the above table.
– 6 – IEC TR 63262:2019 © IEC 2019
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
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the specific document. At this date, the document will be
• reconfirmed,
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IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
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INTRODUCTION
A unified power flow controller (UPFC) adjusts both the active and reactive power of a
transmission line by regulating and controlling line impedance, bus voltage and phase angle
difference. When addressing a lack of power control methods and the insufficient supporting
capacity of dynamic conditions, a UPFC provides an effective solution. Before 2005, there
were three UPFC projects around the world: Inez UPFC project installed in 1998 in U.S.A.,
Kangjin UPFC project installed in 2003 in South Korea, Marcy UPFC project installed in 2004
in U.S.A. (see Annex A).
Ten years later, with relevant technology upgrades and increasing electric power demand,
three more UPFC projects have been constructed and placed into service, all in China. They
are the Nanjing 220 kV UPFC project installed in 2015, Shanghai 220 kV UPFC project
installed in 2017 and Suzhou 500 kV UPFC project also installed in 2017. All these projects
are based on the modular multilevel converter (MMC) technology which has successfully
mitigated the issue of uneven power flow distribution, improved power supply capacity and the
reliability of power supply in related areas. It is believed that with the further growth of electric
power demand, UPFC technology will be more extensively applied in the power marketplace.
This document is based on the practical experience of UPFC projects using modular
multilevel converter (MMC) which is a most perfect type of a voltage sourced converter (VSC)
that can provide technical references for UPFC design, manufacture, test, commissioning,
operation and maintenance.
– 8 – IEC TR 63262:2019 © IEC 2019
PERFORMANCE OF UNIFIED POWER FLOW CONTROLLER (UPFC)
IN ELECTRIC POWER SYSTEMS
1 Scope
This document provides guidelines for applying unified power flow controllers (UPFC) in
power systems. It includes letter symbols, terms and definitions, principles and configurations,
design rules, performance requirements for key equipment, control and protection, insulation
co-ordination, system performance and tests. This technical report applies to the UPFC based
on modular multi-level converter (MMC) technology, as well as UPFC based on three-level
converter technology.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60071-1, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60071-5:2014, Insulation co-ordination – Part 5: Procedures for high-voltage direct
current (HVDC) converter stations
IEC 60076-2, Power transformers – Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-3, Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-4, Power transformers – Part 4: Guide to the lightning impulse and switching
impulse testing – Power transformers and reactors
IEC 60700-1, Thyristor valves for high voltage direct current (HVDC) power transmission –
Part 1: Electrical testing
IEC 61954, Static var compensators (SVC) – Testing of thyristor valves
IEC 62501, Voltage sourced converter (VSC) valves for high-voltage direct current (HVDC)
power transmission – Electrical testing
IEC TR 62543, High-voltage direct current (HVDC) power transmission using voltage sourced
converters (VSC)
IEC 62751-2, Power losses in voltage sourced converter (VSC) valves for high-voltage direct
current (HVDC) systems – Part 2: Modular multilevel converters
IEC 62823, Thyristor valves for thyristor controlled series capacitors (TCSC) – Electrical
testing
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
unified power flow controller
UPFC
equipment which has two (or more) voltage sourced converters (VSCs) sharing common DC
bus connected to the transmission system in parallel and in series, and can control the line
impedance, voltage amplitude and phase angle at the same time
unified power flow controller using modular multi-level converter
MMC-UPFC
UPFC using multi-level converter in which each voltage sourced converter (VSC) valve
consists of a number of self-contained, phase voltage sourced converters connected in series
shunt transformer
transformer which is connected between the converter and the AC grid, in parallel with the AC
power grid
series transformer
transformer which has a winding in series with the line to change the line voltage and/or
phase
Note 1 to entry Other windings such as exciting winding and balancing winding can be chosen by customers.
fast bypass switch
FBS
device connected across the terminals of protected equipment during the turn-off procedure of
the bridge(s) and to transfer current form protected equipment during the turn-on procedure of
the bridge(s) with fast conduction performance during line fault
thyristor bypass switch
TBS
power electronic switch with anti-parallel connected thyristors between the converter and the
series transformer valve-side winding
valve reactor
reactor (if any) which is connected in series to the VSC valve
Note 1 to entry One or more valve reactors can be associated to one VSC valve and might be connected at
different positions within the valve.
– 10 – IEC TR 63262:2019 © IEC 2019
[SOURCE: IEC 62747:2014, 7.22, modified – The words "of the controllable voltage-source
type" have been deleted from the definition, as well as the two last sentences of the note to
entry.]
bypass operation time
time from the occurrence of the fault to the bypass switch being completely closed
multiple valve unit
MVU
single structure comprising more than one valve
[SOURCE: IEC 60633:2019, 6.3.2, modified – The notes to entry have been deleted.]
shunt unit
unit consisting mainly of a shunt transformer and a shunt converter, which achieves the
function of a static synchronous compensator (STATCOM)
series unit
unit consisting mainly of a series transformer and a series converter, which achieves the
function of a static synchronous series compensator (SSSC)
3.2 Symbols
C sub-module capacitance
C VSC DC capacitor
VSC
Tp shunt transformer
Ts series transformer
U line-to-line AC voltage of the converter
a/b/c
U AC voltage of the receiving-end AC system, RMS value
r line-to-line
U AC voltage of the sending-end AC system, RMS value
S line-to-line
U UPFC pre-compensation voltage
ΔU compensation voltage by voltage regulation
U compensation voltage by impedance regulation
c
U compensation voltage by phase angle regulation
α
U DC line-to-line voltage of the DC bus
d
U line-to-ground voltage of AC side of VSCs, RMS value
VN
V upper arm voltage
u
V lower arm voltage
d
X transmission line inductance
Z transmission line impedance
δ sending-end voltage angle
s
δ receiving-end voltage angle
r
4 Principles and configurations
4.1 Basic principles
The UPFC can be equivalent to a voltage source that can adjust amplitude and phase angle
ranging from 0° to 360°.The line current flows through this voltage source, resulting in the
exchange of active and reactive power between the voltage source and the AC line. The
structure of a UPFC used in a two-terminal transmission system is shown in Figure 1.
Figure 1 – UPFC used in a two-terminal transmission system
The active and reactive power of transmission lines are as follows：
UU
sr
(1)
P sin(δ−δ)
sr
X
U
s
(2)
Q= U −U cosδδ−
( ( ))
s r rs
X
The UPFC regulates the line power flow by changing U , U , δ , δ and X. A UPFC power flow
s r s r
schematic diagram is shown in Figure 2. For active power, it is absorbed or generated by the
UPFC shunt converter VSC1 via shunt transformer Tp from the connection point, and is
transmitted via the DC side of the UPFC and series converter VSC2, ultimately delivered to
transmission lines via the series transformer Ts. The UPFC provides an active power
transmission channel for the line, enabling the total active power line transmission capacity to
be increased or decreased. For reactive power, power exchange occurs on the Tp and Ts
through the VSC1 and VSC2. There is no reactive power exchange between VSC1 and
VSC2 [1] .
Therefore, the UPFC is able to control the power flow, changing not only reactive power but
also active power.
The various control functions of the UPFC are briefly illustrated in Figure 3. The UPFC
voltage regulation function is shown in Figure 3 a), where the UPFC series compensation
has the same phase as U or its opposite, only regulating the amplitude of the
voltage ΔU
0 0
voltage, instead of changing the phase of the voltage. Owing to the flexible control of series
output voltages, the UPFC can easily achieve voltage regulation. Series compensation in
UPFCs is the same as general series compensation. As shown in Figure 3 b), the series part
has no active power exchange with transmission lines, so offset voltage U should be
c
perpendicular to the line current I. Figure 3 c) shows a phasor diagram of the phase angle
compensation, which changes the voltage phase angle, but does not change its magnitude.
UPFC compensation voltage is indicated on the arc shown in Figure 3 c). Hence, a UPFC is
equivalent to a phase shifter. Figure 3 d) shows a phasor diagram of UPFC comprehensive
functionality, integrating former three functions, which changes the amplitude and phase of
the voltage according to system operation [2] [3].
——————
1 Numbers in square brackets refer to the Bibliography.
=
– 12 – IEC TR 63262:2019 © IEC 2019
a) Active power flow
b) Reactive power flow
Figure 2 –UPFC power flow schematic diagram
Figure 3 – UPFC control functions
4.2 UPFC configurations
Basic structure
A basic structure diagram of the UPFC is illustrated in Figure 4, consisting of the main circuit
(series unit and shunt unit) and a control unit. The main circuit consists of two VSCs
connected back-to-back on the DC side, and the AC terminals are connected to the systems
via two transformers: VSC1 is connected to the transmission line in parallel via transformer Tp,
and VSC2 is connected to the transmission line serially via transformer Ts. VSC1 and
transformer Tp are the main shunt unit components. VSC2 and transformer Ts are the main
components of the series unit [1] [4] [5].
Figure 4 – UPFC structure diagram
UPFC configuration in single transmission line
In a single transmission line, only one UPFC is installed, as shown in Figure 5. The VSC in
the shunt unit can be connected with the same side AC bus or connected with other AC bus
alone.
The single transmission line UPFC can control the line power flow, regulate the AC voltage in
the shunt unit, or provide fast reactive power compensation.
Figure 5 – UPFC configuration in single transmission line VSC
UPFC configuration in double transmission lines
4.2.3.1 General
Double transmission line UPFCs can be composed of multi-terminal UPFCs or multiple single
transmission line UPFCs with the double transmission line power flow controlled
independently. The basic configuration is divided into non-common DC bus type and common
DC bus type.
4.2.3.2 Non-common DC bus type
The non-common DC bus type UPFC consists of two sets of back-to-back connected VSCs.
The double-line power flow can be controlled by the two series unit VSCs independently.
The non-common DC bus type UPFC has a simple structure as shown in Figure 6. When a
shunt unit VSC fails or overhauls, the corresponding series unit VSC can operate in the SSSC
mode, which is still able to meet the requirements of double-line power flow control and AC
– 14 – IEC TR 63262:2019 © IEC 2019
system voltage control in shunt unit. However, the two shunt unit VSCs need to be controlled
coordinately, and the shunt and series VSCs cannot act as backup to each other, resulting in
the inefficient use of VSCs, which has an impact on reliability.
Key
Tp shunt transformer connected with line1
Tp shunt transformer connected with line2
Ts series transformer connected with line1
Ts series transformer connected with line2
Figure 6 – UPFC configuration with non-common DC bus
4.2.3.3 Common DC bus type
In the common DC bus type UPFC, the VSCs are connected with a common DC bus to reduce
the number of VSCs, as shown in Figure 7. In normal operation, two VSCs are connected to
the series unit to control the power flow of the double transmission lines individually, and one
VSC is connected to the shunt unit to support the line power flow and improve the shunt AC
system reactive power reserve capacity. If a critical shortage of grid reactive power occurs, all
VSCs can be connected to the AC system in parallel to steady the grid voltage.
Therefore, the three VSCs can serve as spares for each other, which makes it easy for the
converter valve to overhaul and isolate the fault area, making the operating mode more
flexible by adjusting the AC switch.
Figure 7 – UPFC configuration with common DC bus
UPFC configuration in multiple transmission lines
The UPFC configuration in multiple transmission lines can be deduced by analogy.
5 Design rules
5.1 Proposal selection
The principles of proposal selection for a UPFC mainly include the following [6].
a) In feasible UPFC schemes, it is necessary to consider the power grid needs for the active
and reactive power control of a UPFC and factors such as site condition and corridor.
Feasible UPFC site schemes should be agreed with customers. In addition to the site
selection rules of conventional substations, for the purpose of UPFC site selection, it is
essential to consider at least the following aspects:
• a region with mature power grid framework but limited land resources;
• a combined view of the present situation and future plans of the power system,
comprehensively considering the control effect and efficiency of running a UPFC;
• a realization of larger flow adjustment with smaller VSCs' capacity.
b) Developing corresponding grid connection proposals for each installation site and
determining a UPFC capacity of each proposal by electrical calculation.
c) Analysing the adaptation of each proposal, including:
– for an uncertain power supply during planning period in service (or decommissioning),
analysing whether a UPFC can satisfy the power system needs for active and reactive
power control.
– for an uncertain transmission and transformation project during planning period,
analysing whether a UPFC can satisfy the power system needs for active and reactive
power control if the project is in operation.
– for important operation mode adjustment that may occur during planning period,
analysing whether a UPFC can satisfy the system needs for active and reactive power
control after operation mode adjustment.
d) Technical and economic comparisons through the unit capacity increasing transmission
capacity, reactive voltage support capability, occupancy of social resources, investment
and annual cost. Detailed information of the Nanjing UPFC project can be found in
Clause A.8.
5.2 Parameter selection and coordination
Parameter selection and parameter coordination mainly include the following.
a) The voltage in the UPFC series unit should be considered the different steady-state
operation modes in a target year and a planning year. Different voltage and power flow
control targets can be obtained by calculation.
b) The rated current of the UPFC series unit should match line current.
c) The capacity of the UPFC series unit equals the product of maximum voltage in the series
unit and rated current and satisfies the power flow control demand.
d) The short-circuit current tolerance level should not be lower than the short-circuit current
tolerance level of the connected system in maximum operation mode. The short-circuit
current duration should match the system short-circuit current duration.
e) The series transformer of the UPFC has over-excitation capacity under maximum
short-circuit current of connected system. Over-excitation duration should match operation
time of by-pass switch and operation time of line breakers if the bypass switch loses
efficacy.
f) An FBS is used to protect the UPFC series unit, for example the thyristor bypass switch
(TBS) combined with a fast mechanical bypass switch (MBS). The operation time of the
– 16 – IEC TR 63262:2019 © IEC 2019
FBS is usually designed according to the engineering system requirements. In principle,
the shorter the operation time, the more effective it is to isolate the fault to protect the
equipment.
g) The capacity of the UPFC shunt unit satisfies requirements of active power exchange of
series unit demands, system reactive power demands, and AC voltage control demands at
the same time.
h) In DC field, determine DC voltage according to the capacity of VSCs, VSC valve-side
voltages of series and shunt transformer.
6 Performance requirements for key equipment
6.1 General
The main components of a UPFC include VSCs, VSC valve water cooling systems, series
transformers, shunt transformers, FBSs, circuit breakers, valve reactors, starting resistors,
current and voltage measuring devices, control and protection equipment, etc. The following
describes only the performance requirements of VSCs, series transformers, shunt
transformers, FBSs and converter valve cooling systems. The performance requirements of
control and protection devices are described in Clause 8. Moreover, other devices can refer to
the corresponding publications.
6.2 Voltage sourced converters (VSCs)
General
The VSC is the key component between the UPFC and the grid for exchanging active and
reactive power. It generally consists of VSC valves and other components. According to the
topological structure, the technical solutions for the UPFC VSC are the three-level multiplicity
converter and the modular multi-level converter.
Three-level converters
The basic topologies of the three-level converters include the diode clamped type three-level
converters and the flying capacitor type three-level converters. Taking the diode clamped type
and IGBT as an example, the main circuit topology is shown in Figure 8. There are two
voltage dividing capacitors in the left side with the same capacitor value. The central point of
the two capacitors is the neutral point, which divides the DC voltage into 3 levels: U /2, 0, and
d
−U /2. See IEC TR 62543 for details.
d
Figure 8 – Typical three-level converter topology
Modular multi-level converters (MMCs)
The typical main circuit topology of a three-phase MMC is shown in Figure 9, which contains
six converter arms. Each arm consists of many sub-modules (SMs) and a valve reactor in
series. The connection point of the two valve reactors is the AC output of the corresponding
phase arm. An SM generally applies a half-bridge structure or full-bridge structure composed
of power electronic switches with a DC storage capacitor in parallel.
Each SM is equivalent to a controlled two-level voltage source, so each arm is equivalent to a
controlled multi-level voltage source. By changing the number of SMs on the upper and the
lower arms, a desired AC voltage output can be simulated. More details are given in
IEC 62751-2. The dotted line is an ideal AC voltage sine wave while the actual generated
waveform is a multi-level step wave, as shown in Figure 10. When the levels are sufficient,
the actual waveform is very close to the ideal sine waveform and has almost no lower
harmonics.
1.1.1 Performance requirements of VSCs
a) Current tolerance
The current tolerance capability of the VSCs should consider the level of the normal
operating current and transient overcurrent for the device, especially for the self-turn-off
switches, capacitors and so on, including amplitude, duration, number of cycles, current
rise, etc. Meanwhile, the sufficient safety margin should also be considered.
Figure 9 – Typical MMC topology
– 18 – IEC TR 63262:2019 © IEC 2019
Figure 10 – Single-phase voltage waveform on the AC side
b) Harmonics
Harmonics are mainly gene
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